Migration to America took long enough for evolution to happen on the way

Similarities in Native American genomes suggest adaptation in ancient history

The Bering land bridge plays a central role in our picture of how humans reached the Americas. When much more of the world’s water was locked up in ice, and the land between Asia and North America was exposed, people followed the bridge to migrate out of Asia, into Alaska, and from there into the rest of the Americas.

This picture tends to portray the bridge as purely a route to the new continents. In fact, the word ‘bridge’ definitely conjures up the wrong image. It was a geographic region, often called Beringia, and people lived there for so long that it probably would have been ludicrous to them that we could think of their home as transient. Current estimates suggest that people lived there for between 5,000 and 8,000 years, starting about 23,000 years ago.

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That is a long enough time for natural selection to have had an effect on the genome of people who lived there, according to a paper in PNAS this week. The Beringians would have faced distinct diseases, food constraints, and climate conditions, and natural selection would have helped those with the right genetic adaptations to thrive in that environment. According to the new paper, we can see evidence of that natural selection in modern Native American populations.

Adaptation to the Arctic

Recently, a genetic survey of 191 Greenlandic Inuit people found some genetic patterns that are so common that the best explanation for them is natural selection. Specifically, there’s evidence to suggest that three genes involved in metabolizing fatty acids (called the fatty acid desaturases, or FADS, genes) show changes that might be the result of adaptation to a diet high in protein and fats. That sort of diet tends to be one of the side-effects of living in the Arctic.

But these conditions aren’t really particular to Greenland; they were probably similar in Beringia. It's possible that the adaptations took place on Beringia itself—in which case they would predate the peopling of all the Americas.

To test this hypothesis, a group of researchers compared the genomes of Native Americans to people from Africa, Europe, and East Asia. In line with earlier evidence, they found variants in the FADS genes that were much more common in the Native American genomes. This is true even though diets among Native American populations became quite diverse over their history.

There’s a growing pile of evidence that the FADS genes are pretty important, says Rasmus Nielsen, who wasn’t involved with this paper, but was one of the authors on the paper about the Greenlandic Inuit genome. The same genes seem to show signs of natural selection in lots of different human populations, and it all seems to have something to do with the histories of what those populations have eaten. “These are genes that seem to be really, really important when the diet changes,” he says.

The founder effect

One tricky thing is that natural selection isn’t the only thing that brings about differences between genomes. When you have a large population of people, they have quite a bit of variation in their genomes—for instance, a population might have some people with brown eyes, some with green, and some with blue.

If a small part of this group breaks away and moves off, they take only part of that total variation with them. Because of random chance, 80 percent of that group of migrants might have brown eyes. If their descendants tend to have brown eyes, that doesn’t mean that natural selection made it that way.

So, if Native American genomes tend to show a lot of differences on the FADS genes, it’s important to check that the result isn’t because of this founder effect. Testing this means looking at the rest of the genome, estimating how strong the founder effect is, and seeing whether the variants in question are more common than you would expect based on a founder effect. When the researchers did this, they found a number of variants that were common enough that natural selection seemed like the best explanation.

Beringia was inhabited by a population, not individuals. So, as you might imagine, not all of the Native American genomes were uniform in terms of which FADS variants they had. Some regions showed more of the variants, and some showed less. “The high frequencies occur despite marked differences in lifestyles and diets of the different indigenous populations,” the authors write. However, there hasn’t been a huge amount of time for natural selection to operate. Future studies could test whether there are any differences among Native American groups that could be connected to their historical diets.

If the evidence keeps piling up that the FADS genes affect how we process our food, they could ultimately be important in medical research, says Nielsen. There’s so much debate about what diet is the most healthy, he says, but we could be looking at the reason why that question doesn’t have a simple answer: the best diet might be different for different people, depending on their genetics.